In the rocks extending from the Lower Silurian and perhaps from the Upper Cambrian to the Devonian inclusive, the type and function of Eozoon are continued by the Stromatoporæ, and in the earlier part of this time these are accompanied by the Archæocyathids, and by another curious form, more nearly allied to the latter than to Eozoon, the _Receptaculites_. These curious and beautiful fossils, which sometimes are a foot in diameter, consist, like Archæocyathus, of an outer and inner coat enclosing a cavity; but these coats are composed of square plates with pores at the corners, and they are connected by hollow pillars passing in a regular manner from the outer to the inner coat. They have been regarded by Salter as Foraminifers, while Billings considers their nearest analogues to be the seed-like germs of some modern silicious sponges. On the whole, if not Foraminifera, they must have been organisms intermediate between these and sponges, and they certainly constitute one of the most beautiful and complex types of the ancient Protozoa, showing the wonderful perfection to which these creatures attained at a very early period. (Figs. 46, 47, 48.) I might trace these ancient forms of foraminiferal life further up in the geological series, and show how in the Carboniferous there are nummulitic shells conforming to the general type of Eozoon, and in some cases making up the mass of great limestones.[AL] Further, in the great chalk series and its allied beds, and in the Lower Tertiary, there are not only vast foraminiferal limestones, but gigantic species reminding us of Stromatopora and Eozoon.[AM] Lastly, more diminutive species are doing similar work on a great scale in the modern ocean. Thus we may gather up the broken links of the chain of foraminiferal life, and affirm that Eozoon has never wanted some representative to uphold its family and function throughout all the vast lapse of geological time. [Footnote AL: _Fusulina_, as recently described by Carpenter, _Archæodiscus_ of Brady, and the Nummulite recently found in the Carboniferous of Belgium.] [Footnote AM: _Parkeria_ and _Loftusia_ of Carpenter.] NOTES TO CHAPTER VI. (A.) Stromatoporidæ, Etc. For the best description of Archæocyathus, I may refer to _The Palæozoic Fossils of Canada_, by Mr. Billings, vol. i. There also, and in Mr. Salter's memoir in _The Decades of the Canadian Survey_, will be found all that is known of the structure of Receptaculites. For the American Stromatoporæ I may refer to Winchell's paper in the _Proceedings of the American Association_, 1866; to Professor Hall's Descriptions of New Species of Fossils from Iowa, _Report of the State Cabinet, Albany_, 1872; and to the Descriptions of Canadian Species by Dr. Nicholson, in his _Report on the Palæontology of Ontario_, 1874.